Semiconductor power devices are used as inverters, DC/DC converters, and other power conversion devices. Semiconductor power devices are primarily used in the form referred to as “power semiconductor modules” in which a plurality of semi-conductor power devices is mounted.
The structure of a typical conventional power semi-conductor module is described below with reference to FIGS. 4 and 5 hereof. FIG. 4 shows a cross-sectional structure of the power semiconductor module, while FIG. 5 shows an electric circuit built into the power semiconductor module.
Examples of such semiconductor power devices include transistors in which ON/OFF operation is controlled by an external signal, diodes having rectifying characteristics, and other devices. Typical transistors include MOSFET, IGBT and the like.
FIG. 5 is an example of an inverter circuit for converting direct current to three-phase alternating current. The inverter circuit 100 comprises at least six IGBTs 101 and six diodes 102. The six IGBTs 101 are connected so as to form a bridge circuit. Each diode 102 is connected between the collector and emitter of the corresponding IGBT 101, with the forward direction being the direction that faces from the emitter toward the collector. The upper terminal 103a and lower terminal 103b on the left side of FIG. 5 are direct-current input terminals, while the three terminals 104a, 104b, and 104c on the right side of the Figure are output terminals for three-phase alternating current. When the power to be controlled is increased in the inverter circuit 100, the amount of heat produced in the semiconductor power devices is increased. Therefore, the heat must be adequately released to the outside to prevent the temperature from excessively increasing in the semiconductor power devices.
In the power semiconductor module 201 shown in FIG. 4, the IGBTs 101 and diodes 102 are generically shown as semiconductor power devices 202. These semiconductor power devices 202 are joined by using solder on a circuit board 210 that has a metal substrate electrode 211, an insulation substrate 212, and a metal substrate 213, which are layered together. A plurality of the metal substrate electrodes 211 is disposed in correspondence with the semiconductor power devices 202. Aluminum wires 222 are connected to the surface electrodes of the semiconductor power devices 202 and are connected to external electrodes 223. In the circuit board 210, the metal substrate electrode 211 and the metal substrate 213 are composed of aluminum, the insulation substrate 212 is composed of aluminum nitride, and these elements are connected to each other.
The role of the metal substrate electrode 211 is to transmit to the exterior at low loss considerable electric current that flows to the semiconductor power devices 202. A material having electrical conductivity is suitable for the metal substrate electrodes 211. Copper and aluminum are mainly used for the metal substrate electrodes 211. The role of the insulation substrate 212 is to assure electrical insulation between each of the metal substrate electrodes 211 and the metal substrate 213. The insulation substrate 212 also serves to bring heat generated in the semiconductor power devices 202 to the exterior. For this reason, a material having high insulation resistance and high thermal conductivity is needed as the insulation substrate 212. Aluminum nitride, silicon nitride, alumina and the like are commonly used for the insulation substrate 212 material.
The metal substrate electrode 211 and insulation substrate 212 are commonly joined by brazing at a high temperature of about 600° C. or higher. When cooled to room temperature after brazing, warping occurs due to stress caused by the difference in the coefficients of thermal expansion between the metal substrate electrode 211 and insulation substrate 212. The metal substrate 213 disposed in a position on the opposite side of the metal substrate electrode 211 by way of the insulation substrate 212 is used for inhibiting such warping, and the three elements in the circuit board 210, i.e., the metal substrate electrode 211, insulation substrate 212, and metal substrate 213 are joined in the same brazing step.
The entire circuit board 210 is joined to a copper base plate 225 by way of solder 224. The constituent elements of the circuit board 210 have a thickness of only 1 mm or less while the base plate 225 has a thickness of several millimeters or more. Also, the solder 224 is flexible and easily spreadable, and serves to reduce thermal stress produced between the base plate 225 and circuit board 210. The base plate 225 is connected to an aluminum heat sink 227 by way of silicone grease 226.
A resin case 228 is fixed on a top of the base plate 225. The external electrodes 223 are fixed to the resin case 228 and extend from inside the case to the exterior of the case.
When the power semiconductor module 201 is operating, a large amount of electric current flows to the semiconductor power devices 202, thus producing heat. When the power semi-conductor module 201 is viewed as a whole, the locally produced heat is transmitted from the circuit board 210 to the base plate 225, is widely dispersed in the lateral direction inside the base plate 225, travels throughout the base plate 225, and is ultimately released into the atmosphere by way of the heat sink 227. The upper-limit temperature of the joined areas of the silicon semiconductor power device is commonly about 150° C. Therefore, a heat-releasing structure is designed for the power semiconductor module so that the mounted semiconductor power devices 202 are not heated above the upper-limit temperature.
The performance of the silicon-based semiconductor power devices 202 has substantially reached theoretical limits. Semiconductor power devices that use silicon carbide (SiC) (herein-after referred to as “SiC semiconductor power devices”) have received attention in recent years as an alternative. SiC semiconductor power devices can reduce loss and operate at a higher temperature than silicon semiconductor power devices. For this reason, heat generation is reduced, high-temperature operation is made possible, and the volume of the heat sink can be reduced by enabling the temperature difference between the heat sink and the outside air or the coolant to be greater in power converters and power semiconductor modules that use SiC semiconductor power devices. There are excellent possibilities for the application of SiC semiconductor power devices as very useful means for reducing the size of power semiconductor modules and power converters.
However, a conventional mounting structure cannot be applied to high-temperature operation that exceeds 200° C., which is the temperature range at which best use is made of the characteristics of a SiC semiconductor power device. Assuming that a power semiconductor module obtained using a conventional mounting technique is employed or stored in a high-temperature environment that exceeds 200° C., or in an environment with considerable variation in the minimum and maximum temperatures, the power semiconductor module will experience a critical failure because heat stress caused by the difference in the coefficients of thermal expansion between the constituent materials becomes excessively high, and the solder and other materials themselves have insufficient heat resistance.
Thus, conventional techniques for mounting semiconductor power devices are practical in the operating temperature range of silicon semiconductor power devices, but fail to provide sufficient ambient heat resistance to devices that operate effectively at higher temperatures, such as SiC semiconductor power devices. In the case that silicon semiconductor power devices are to be used, a mounting technique is needed that produces less heat-induced resistance and provides greater ambient heat resistance in order to make sufficient use of the characteristics of such devices.
Reference is now made to JP 09-148491 A1 and JP 2000-216278 A1 (U.S. Pat. No. 3,479,738), JP 2000-332170 A1 and JP 10-289968 A1 showing conventional techniques relating to the present invention.
The power semiconductor substrate disclosed in JP 09-148491 A1 comprises an insulating board made of AlN (aluminum nitride) and highly-heat-radiant composite material boards made of CuMo (copper molybdenum) and laid on front and rear surfaces of the insulating board. This document shows the substrate only and has no reference to the arrangement including a heat sink. The disclosed arrangement provides improved electrical conductivity and insulation within the power semiconductor. However, the document fails to give consideration to heat radiation.
JP 2000-216278 A1 discloses a semiconductor package which is comprised of an insulating board made of AlN (aluminum nitride) and highly-heat-radiant CuMo (copper molybdenum) composite material boards laid on front and rear surfaces of the insulating board, similarly to the arrangement of JP 09-148491.
JP 2000-332170 A1 discloses a semiconductor device which comprises a base plate made of copper (Cu) with a ceramic substrate bonded to one surface thereof. A molybdenum plate, having a coefficient of thermal expansion similar to that of the ceramic substrate, is bonded to the other surface of the base plate in positional correspondence with the ceramic substrate.
JP 10-289968 A1 is directed to a power semiconductor having an AlN board with non-wired electrically conductive patterns formed around the board. This arrangement makes it possible to suppress generation of a stress in the surface of the AlN board by virtue of balanced control of the stress generation in the front and rear surfaces of the board.
Thus, conventional techniques for mounting semiconductor power devices are practical in the operating temperature range of silicon semiconductor power devices, but fail to provide sufficient ambient heat resistance to devices that operate effectively at higher temperatures, such as SiC semiconductor power devices. In the case that silicon semiconductor power devices are to be used, a mounting technique is needed that produces less heat-induced resistance and provides greater ambient heat resistance in order to make sufficient use of the characteristics of such devices.
In the structure of the power semiconductor module shown in FIG. 4, the pathway from the semiconductor power devices 202 to the outside air is a complicated structure that begins at the solder 221; passes through the metal substrate electrode 211, the insulation substrate 212, metal substrate 213, solder 224, base plate 225, and silicone grease 226; and ends at the heat sink 227. This resulted from the object to alleviate the thermal stress produced by the difference in the coefficients of thermal expansion between the constituent materials and from the restrictions encountered in the process of manufacture.
Therefore, when power semiconductor modules are configured using a SiC semiconductor power device, the modules need to have a structure that has a simpler pathway from the semiconductor power devices to the outside air, and to have sufficient ambient heat resistance with respect to SiC semiconductor power devices that operate effectively at higher temperatures.